1. Field of the Invention
The present invention relates to radio-frequency (RF) circuitry and, more particularly, to apparatus for directly measuring the values of components within an RF circuit.
2. Description of the Background Art
A plasma enhanced semiconductor wafer processing system contains a vacuum process chamber within which certain plasma enhanced processes are performed upon a semiconductor wafer. To produce a plasma within the process chamber, a reactant gas is pumped into the chamber and a high-power RF signal is coupled to the gas. The RF energy excites the reactant gas and produces a plasma within the chamber which is used to process a wafer supported within the chamber atmosphere.
To optimally couple the RF energy to the plasma, an RF matching network is used to match impedance of the RF source to the impedance of the chamber atmosphere. The RF matching network generally contains a plurality of tunable elements such as variable inductors and variable capacitors arranged in, for example, a .pi.-network. The matching network elements are generally set to predefined values prior to applying the high-power RF signal to the matching network. The predefined values produce an approximate match for the RF signal to the chamber atmosphere. Since the impedance of the chamber atmosphere is time variant, the RF matching network must be dynamically tuned to maintain the impedance match. As such, once the plasma is excited, the RF matching network is tuned to optimally achieve a match between the RF signal and the plasma. Additionally, the matching network requires tuning as chamber conditions change during wafer processing. Tuning is generally accomplished by mechanical means such as a motor and gearing arrangement to adjust inductive and capacitive tuning elements within the matching network.
The actual values of the matching network components are generally unknown; however, the position of the tuning elements within the capacitor and/or inductor used to tune the matching network are known using a position feedback technique. Typically, a position feedback system mechanically couples a potentiometer to the tunable element and the resistance of the potentiometer varies with position of the tuning element. As such, the resistance is indicative of the tuning element position and the value of the tunable element.
Although accuracy and reliability of the indirect measurement method is limited by the calibration and physical robustness of the electromechanical components, the indirect method is widely used to measure the tuning element values because the environment within the RF matching unit is harsh and prone to generate substantial amounts of signal noise that interferes with direct component measurement. Such noise degrades any signals used to directly measure the value of the tuning elements.
Therefore, there is a need in the art for apparatus that directly measures the value of the tuning elements within an RF circuit and, in particular, within an RF matching network of a semiconductor wafer processing system.